Density
Density describes the number of individual plants in a given area. In situations where identification of individuals is ambiguous, density measurements may be based on some other counting unit, such as culms or shoots for sod-forming grasses or the basal stems for shrubs.
Units to express density should be selected so that actual plant numbers are easy to visualize. For example, it is more meaningful to express an infrequently occurring shrub as 50 shrubs/ha rather than 0.005 shrubs/m2, whereas 15 seedlings/m2 is easier to imagine than 150,000 seedlings/ha for the density of an abundant annual grass.
Density is an attribute that is tedious to measure but easy to interpret. It is most often applied to larger plants, such as tress, shrubs, and more prominent perennial forbs. Density is often used as a baseline inventory of the structure of rangeland or forest vegetation, by quantifying different species or various ages within a single species. Density data is also gathered to monitor the effect of various land use treatments, such as plant survival following burning or herbicide application, particularly for woody species.
Density measurements are sometimes unsuitable for the herbaceous layer, especially when there are numerous plants to count or identification of individuals is difficult. Nonetheless, density is regularly used to evaluate seedling emergence and survival in a rangeland reseeding program. Density is also commonly described in autecological studies, that trace the demography of herbaceous or woody populations.
Species composition can be determined from density data, by calculating the contribution of each species to the total plant count of the sample. However, the relative density of species of differing sizes provides little information about their actual biomass, ecological dominance or community function. For example, the carpets of annual poppy seedlings that establish following cool season rains in the Sonora Desert may have a high density, yet contribute little to the biomass compared to a less dense, but larger species such as triangle leaf bursage (Ambrosia deltoidea). For this reason, expressions of species composition estimated from density data are usually restricted to a particular life form, such as the shrub layer or herbaceous layer.
Density can provide useful indicators in an inventory and monitoring program to determine range condition and range trend because it remains relatively stable from year to year, regardless of changes in biomass that result from rainfall fluctuations or short-term grazing patterns.
Terminology Related to Density
The following term is associated with sampling to estimate density.
Special Considerations for Density Sampling
Here are some critical issues to be considered when designing sampling protocols to determine density:
- Boundary Decisions
- Identification of Individuals
- Influence of Spatial Patterns
- Rooted or Canopy Dimensions
- Species Groups
Measurement Techniques and Statistical Analysis
References and Further Reading
(Note: pdf files require Adobe Acrobat (free) to view)
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp 127-197.
Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 61-63.
Cooper, C.F. 1959. Cover vs. density. Journal of Range Management 12:215. (pdf)
Daubenmire, R. 1968. Plant communities: A textbook on plant synecology. Harper Row, New York, NY. pp 40-41.
Greig-Smith, P. 1983. Quantitative plant ecology. Blackwell Scientific Publications, Oxford.3rd ed. pp 26-33.
Laycock, W.A. 1985. Density as a method for measuring rangeland vegetation. In W.C. Krueger (Chairman) Symposium on use of frequency and density for rangeland monitoring. Proceeding 38th Annual Meeting, Society for Range Management, Salt Lake City, UT, February 1985. pp 91-100.
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and methods of vegetation ecology. John Wiley Sons, New York. pp 68-70.